Based on my initial analysis, this seems to be a work in progress and more functions will be added over time. This is some sample code how you can do the basic CRUD using this new feature. This is not an exhaustive documentation, but considering that there is nothing about this in the official documentation, it is a starting point.

You can use this on your client side code on v9. It is quite basic at the moment, but you don’t need to include any external libraries. But in more advanced scenarios, you can always use Xrm WebAPI Client till these features are made available in the Client API.